Field of the Invention
The present invention relates to an image forming apparatus adopting an electro-photographic recording system such as a copier, a facsimile, a laser beam printer, a multi-function printer, and the like.
Description of the Related Art
Lately, there is produced a low-cost and space-saving image forming apparatus adopting an electro-photographic system such as a copier, a facsimile, a laser printer, a multi-function printer, and the like. Such image forming apparatus is widespread not only in offices but also in small offices and in homes for personal use, thus creating an environment of enabling to self-manufacture a small amount of various kinds of prints such as leaflets, bills, catalogs, and the like. Under such circumstances, the image forming apparatus is naturally required to be able to enhance image quality and to be highly adaptable to various kinds of sheets.
What is demanded most among the various kinds of sheets is a coated sheet, i.e., a sheet whose smoothness and appearance are enhanced by coating a surface of a high quality sheet. Because the coated sheet is glossy, highly smooth, can clearly reproduce a photograph and characters, and provides high quality finish as features of the coated sheet, the coated sheet can be suitably used for leaflets, bills, catalogs and others described above.
Meanwhile, the coated sheet has a problem that a surface layer face of the coated sheet is liable to absorb moisture and the coated sheets overlapping with each other are liable to adsorb with each other if a bundle of coated sheets is left under a highly humid environment. Due to that, the coated sheet is liable to cause such problems as double feeding of conveying multiple coated sheets while overlapping with each other and as feed failure of not conveying any sheet.
Then, conventionally, Japanese Patent Application Laid-open No. Hei.11-157686 proposes a technology of restraining the adsorption of the sheets by blowing air to side faces and an upper face of the sheets stacked on a sheet stacking portion to separate the sheets in an image forming apparatus.
However, there is a case when the double feeding occurs because overlapping sheets adsorb with each other by an electrostatic force, beside the adsorption caused by humidity absorbed in a highly humid environment, because smoothness of the sheet face of the coated sheet is high in particular. The sheets stacked on the sheet stacking portion are put into a charged state in the electro-photographic system in particular because high voltage is applied to the sheet to transfer a toner image onto the sheet in a transfer portion and the transfer current flows into the sheets stacked on the sheet stacking portion.
In a case where a distance between the transfer portion and the sheet stacking portion cannot be fully assured due to downsizing of the apparatus in the electro-photographic system image forming apparatus, a sheet on which a toner image is to be transferred in the transfer portion is in contact with a sheet on the sheet stacking portion such that parts thereof overlap with each other. Still further, an entire resistance value of the sheets stacked on the sheet stacking portion decreases in cases when a resistance value of a material of the sheet is small or an amount of the sheets stacked on the sheet stacking portion is small in the condition in which the sheet stacking portion is earthed. Under such circumstance, a potential difference between the sheet being charged during the transfer and the sheet stacked on the sheet stacking portion increases, thus generating an electrostatic force. The double feeding occurs because the sheets adsorb with each other by this electrostatic force (electrostatic adsorption). Then, if the adsorption between the sheets caused by the electrostatic force is strong, it is unable to separate the sheets even by a sheet separating operation of the sheet feeding portion, and the sheets are conveyed to the transfer portion while being overlapped. Such circumstance occurs remarkably in the case of the coated sheet.
It is unable to separate the sheets adsorbed with each other by the electrostatic force by the configuration of blowing air as described above.
Then, it is conceivable to prevent the double feeding caused by the electrostatic force by suppressing the potential difference between the sheets by enhancing insulating performance of the sheet stacking portion. However, electric charge is gradually accumulated in the sheets on the sheet stacking portion and in the components in the process of consecutively conveying the sheets. Then, if the charge exceeds a certain threshold value, there is a possibility of generating electrostatic noise, thus causing such problems as erroneous operations of electronic components within the image forming apparatus.